Spontaneous formation of two-dimensional wrinkles in poly(urethane-isocyanurate) rigid foam boards

Processing low density poly(urethane-isocyanurate) rigid foam boards still remains challenging, especially for high thickness panels. In particular, subsequent shrinkage of the foam panel during cooling and cross-linking of the matrix creates in-plane compressive stresses in the facing that can be relieved by buckling with a relatively uniform wavelength of a few millimeters. An extensive study in the scientific literature documents the variety of wrinkling morphologies obtained when a thin film bonded on top of a soft substrate is compressed, and we proposed to apply this model in order to explain experimental observations of wrinkling in the case of a flexible facing bonded to a poly(urethane-isocyanurate) rigid foam. The wavelength, amplitude, and pattern of the waves were found to give information about the mismatch in Young's modulus as well as the level of compressive strain in the foam panel at the moment of wrinkling formation. A shore durometer (type 00) was systematically used to quickly probe the mechanical properties of the foam surface at various locations along the width of the insulation panel (after removal of the facing) and was found to be a valuable tool to explain the location of surface wrinkling and its evolution with time. Variations in mechanical properties of the foam surface were explained in terms of differences in cell orientation near the surface.